Hacker News Reader

The trick is that heat pumps are using energy to move heat. Let's make an example calculation:

When heating your house to burn coal, you release 1 Joule when burning the coal. It's a small plant, so it is not very efficient. You probably get 0.85 J of heating out of it.

If we instead burn coal in a large-scale modern power plant, we release the same 1 Joule, but capture 0.95 J of it in electricity. Let's say we lose 10% transporting it to your home, leaving 0.855 J remaining. Now, we use that energy to move heat from outside into our home, at an efficiency of about 300% - so for every joule we consume, we move 3 joules of energy. This means our house is heated with 2.565 J. But the energy the heat pump consumes doesn't disappear - it becomes heat too. So your house gets a total of 3.42 J hotter!

So 1 J of coal gets you either 0.85 J of heat by burning it directly, or 3.42 J of heat by burning it in a power plant and using a heat pump. And of course don't forget that for climate reasons it is trivial to replace that power plant with a solar farm or wind turbine.

The worst-case scenario for a heat pump is that it is exactly as efficient as burning stuff directly. But in virtually all cases, it is way better.

Pretty good assessment, except I think your figure for heat efficiency of power generation is way off. I would expect to convert around 30-40% of the heat energy to electricity.

95%+ efficient coal boilers have existed since the '90s.

This is fuel to electricity efficiency?

That seems unlikely to me, both based on sources[1][2], and the thermodynamics of the Carnot equation. A Carnot cycle heat engine (which is most efficient possible AIUI) has a maximum efficiency of 1 - Tc/Th. Given a Tc of 300K (STP-ish), achieving an efficiency of 0.95 would require a Th of 6000K, well above the melting point of steel (only around 1800K).

1: https://www.eia.gov/todayinenergy/detail.php?id=44436

2: https://www.energy.gov/fecm/science-innovation/office-clean-...

You seem to have only half understood?

The heat pump is only moving the heat. Burning coal doesn't move heat, it makes heat by destroying coal. Because the heat pump isn't making the heat it is able to move the same amount of heat from somewhere else (say, the outside air) into your home, using less energy than you'd need to make that heat inside your home.

Because you can't destroy heat (thermodynamics) for refrigerators we have no choice, we must use heat pumps. But once you get good at this technology, it's just better regardless of whether you want to move the heat away (Air conditioning, refrigerator) or towards you (heating a home).

This is how heat pumps claim seriously > 100% efficiency in many cases, they're comparing the efficiency of moving heat against making it.

It's important for people to know that at least when it comes to heating, "100% efficiency" is actually the worst possible efficiency. That's just thermodynamics. You can only get better than that.

No, you can get much worse as well when considering heating in general.

Heating efficiency is a measure of useful heat, so while of course energy is conserved in the universe, it is not 100% efficiency as measured, where we consider the consumed energy versus the successful heating of the home. Heat lost to the outside is inefficient.

With natural gas heating, efficiency used to be around 50% as the exhaust gases were quite hot. They just had passive chimney flues driven by natural convection. Improvements to recover more of that heat brought efficiency up to 80% and now approaching 95% in the most modern configurations that I've read about. They now require mechanical ventilation to remove the exhaust and also have condensate drains because the heat exchangers cool the gases so much.

Of course, something like a traditional wood fireplace is much worse than even those old gas furnaces, with a vast amount of energy going out the chimney.

Sorry, I meant to say electrical heating!

With electrical heating, if you consider transmission losses you can see that the effective home heating is less than 100% of the generated power. The power lines and equipment warm the outside too.

Another benefit of the high COP heat pumps: in reducing the total power consumption at the home, it also brings a proportional reduction in those losses outside the home...

While this is true in some sort of physical sense, it's not really useful for home heating. The problem is that while burning stuff makes heat, that heat probably isn't where you wanted it, and you need to capture the heat and move it to where it's useful. Unfortunately burning stuff produces exhaust gases, and those too are hot, that is they contain some of the valuable heat, but the gases themselves are dangerous and unwanted.

Modern boilers achieve about 94% efficiency. That is, if we make 1MJ of heat energy by setting stuff on fire, the boiler transfers 940 kJ of heat energy into hot water. The remaining 60 kJ is warm exhaust. You've probably seen the exhaust from people's homes if you live somewhere with such boilers.

Heat pumps produce several times the heat as electricity put in. Even with a typical 40%-50% efficiency for a gas power plant you end up with more than 100% efficiency with a heat pump. If the power plant rest heat is used for instance for district heating you reach efficiency of around 90%. Add the efficiency of heat pumps on top and you end up with something very good.

What? No, this is incredibly short-sighted. It overlooks the fact that much grid energy is renewable already and will become more so in the future. Go with electric sources for cooking and heating (home and water), then let the grid handle being renewable.

Heat pumps are amazing technology and moving heat, vs generating it, is incredibly efficient and clean.

Do not burn fissile fuels in your home.

> Do not burn fissile fuels in your home.

Completely agree; that would be incredibly dangerous and polluting (and probably illegal)!

Buffy had it right, "Fire bad, tree pretty".

Heat pumps transfer heat from the outside, they aren’t generating it so it’s entirely likely that they would result in fewer net emissions than burning coal directly. And the calculus just keeps getting better for grids that incorporate more natural gas and renewables.

Typical heatpump COP in Northwestern Europe sits at about 4, that means for 1 kWh of electricity your moving 4 kWh heat into your home. Electrical resistive heating sits at 1:1 by definition, and methods like wood, gas or oil, combustive methods, usually with modern installations at 80-95% efficiency. Further south heatpump COP improves; the rule of thumb is a smaller delta T the better.

Moving heat is always more efficient than creating heat. It's elementary physics.

Airconditioners, or air-to-air heatpumps, are cheap heaters for the same reason (any units sold in the last 10 years can move heat bidirectionally). Here in NW Europe by cooling bill is barely noticeable (it's no desert clime after all, although the last decade seems like it's getting there!). You don't need to set it to 18C; I really only cool when the attic gets over 28C while I work there (home office), down to 24-26, which is a few weeks a year. In winter, it heats the room at ~400% efficiency compared to the typical ~80 for gas heating.

Move heat. Don't create it.

> Typical heatpump COP in Northwestern Europe sits at about 4

:) yeah, when its really quite warm outside, but not quite warm enough that you dont need heating.

as soon as you even get down to 5C outside, you are not getting a COP of 4. Many vendors may tell you that, but well... :) some people were also sold bridges :)

A lot of people in my environment are in fact getting sCOP of 4. Bit better in fact: close to 5 in the group that also installed floor heating everywhere.

Note that COP around freezing is almost always the worst due to frequent defrosting, so yes, COP is worse at -5 most likely. You can replace COP with sCOP in earlier replies of mine.

What is true is that most people, including installers, don't know how to tune them very well, or forget to combine them with low temp radiators/ floor heating. Too high a T_out also occurs frequently.

In the Netherlands, heatpumps have been standard issue in new construction for a decade now, and 4 is about the average. It's very much not imaginary, but hard to believe for people from areas used to wood and oil burning ;)

i would question whether that is in fact happening. MAYBE scop, for the recent winters where it has not been cold, but unless you have a vastly overdimensioned setup for what is done now then, you'll be screwed if a real winter happens again

Overdimensioned: that can be a cause for low COP, and not always a surefire solution. But yes, properly dimensioning to your particular situation is more important than with other heating methods, agree. The best point I think is to target not the coldest winter you'd ever expect, because that means suboptimal runtime the rest of the year. Better is to target optimal runtime for most of the year, and accept horrible performance for a few days every 5 years or so.

Modern heatpumps work perfectly fine down to -20C or so, which covers nearly all of Europe.

If your winters are around 0C, it really pays to get units with better defrost management. You can use power output ratings; cheaper brands might have a third less output at 0deg, while others barely dip. In colder countries the cheaper units are funnily enough a great option, because once it's cold enough to continuously flow, you don't have to defrost at that often anymore.

they might work at -20C, but you cannot get ANYWHERE near the output out of them as you can when its hotter.

Several of my family members have heatpumps, myself included, and I can tell you that the COP reduces significantly as soon as we hit just 0C outside. my panasonic has a supposed SCOP of 5.1(or 5.2, cant recall), when it hits 0C here, its down to COP of ~2.5 at my best estimates.

when it hits -10, the COP is at best 2. I know this because I think around 1300W into it, and it is not able to heat my house more than a 2.5kW resistive heater.

this is their 3 year old flagship "nordic heat pump" that they will market as the sole heating solution for houses twice the size of mine, yet can barely do mine when it hits -5-10C :)

edit: im not saying heatpumps are bad, they are great, but I dont think they are a great ONLY solution for places that are below 0C for more than tiny peak times.

when its 5-10C outside, and you want 22C inside, they are really amazing

Like I said, around 0 you're gonna get the worst performance, as outside temp goes down most models will see they COP actually increase (compared to ~0 C). Of course, delta-T is still important, so it helps hugely if you can have your water temp at 30 C (and still radiate effectively). It takes a carefully planned systems to reach that though (i.e. active ventilation on your radiators, or floor heating), so many will need to get water out at 40 C or even higher, which is going to bring you down from sCOP 4. At -20 COP is going to be 2-2.5. But, for many Europeans, that's not a frequent event. Even so, 200% efficiency is better than <100%.

I think many don't put much effort in the radiator side of things. E.g. here gas heating is usually ran at 60-70 degrees, while for that sCOP of 4+ it should be 30-35 C. Roughly, you're going to need a factor 4 more radiating surface. People don't always install that, and then run their heat pump at 50 C and get a COP of 2. Well, no shit :)

So, you gotta take a reasonable low point, understand how hot your circuit temp is going to be (the formula for radiators is pretty much the same over radiator brands from those I've used, floor heating idem), account for buffer losses (with floor heating, you can cut out your buffer and gain 0.5 COP just like that), and, if you have an outdoor temp of ~0 C for much of the year, look for a model that comes with a good defrosting strategy and keeps an acceptable efficiency around that temp. sCOP 4 is for the Netherlands, which has mild winters thanks to the Gulf Stream. However, family is in Poland, and there heat pumps are now also being installed everywhere (where below -20 C is expected for weeks each winter) and while you're right that this means an sCOP reduction, it can still be near 3 if you configure the rest of the loop right. And 250-300% efficiency is going to beat resistive heatings 100% or combustive methods each and every time.

2.5 kW sounds severely underdimensioned for a house at -20 C though. That would not cut it for my smallish house in NL, I've calculated ~5 kW at -15 C needed (and don't forget, make sure your radiators can emit this at the T_out!). Typically people would install a 7-10 kW unit for my sort of house.

> Like I said, around 0 you're gonna get the worst performance, as outside temp goes down most models will see they COP actually increase (compared to ~0 C)

This simply makes no sense, and is not to be found in ANY datasheets from any of the big manufacturers. How do you propose this will work? a given volume of air, say 1 cubic meter, has a certain amount of heat at 0C, if you take it down to -5C or -10C, that amount of heat is reduced. The heatpump can only put so much air over the heatsink, and thus the absolute amount of heat visible to the heatpump reduces as temperature goes down.

> 2.5 kW sounds severely underdimensioned for a house at -20 C though

well yeah, I have of course not installed such a heatpump (with a max draw of ~1500W in plus degrees) as my only heat source, im just trying to convey the output capabilities

> This simply makes no sense

It makes total sense, if you understand what a defrost cycle is. In the LG sheet I have in front of me its quite clearly reflected in the effective power output: a dip around zero.

that makes no sense. the coil will easily dip below 0C even at >0C outside, and need to defrost.

You're not the first havng a hard time wrapping their heads around this, but you'll get there ;)

replying here as could not reply deeper in thread...

Well maybe you ought to enlighten mitsubishi and panasonic, because they do not appear to have wrapped their minds around this yet. Looking at chinese manufacturers, neither have they.

The argument is that for regions that do not spend much time below 15 degrees Fahrenheit, they tend to be cheaper to run because they can pump more heat than electricity they use, effectively becoming 300% efficient instead of 95% efficient for say an gas heater.

As the temperature differential between the desired temperature and the outside temperature increases however, the efficiency does drop and what you say is true, but for most of the populated world, this is not the situation.

https://www.youtube.com/watch?v=7J52mDjZzto

(edit: I think you're going to get a lot of links to the overly in depth series tech connections, but I'm leaving it here all the same)

Heat pumps can be efficient enough to overcome the losses from coal -> electricity. So it's possible to use a heat pump to heat a space more efficiently than burning that same coal directly in the space.

>Burning something for heat is always more effective, cheaper, and produces fewer emissions compared to an electric heater... >If you’re getting electric heat from coal fired steam that’s always worse than burning the coal locally to heat something, and this is true for all emission-producing power sources.

Apparently you can actually save natural gas with electric heat and a heat pump.

https://youtu.be/MFEHFsO-XSI